Abstract

Time-resolved absorption spectroscopy is often rejected in favor of laser-induced fluorescence spectroscopy as a viable technique for the investigation of ultrafast photochemical phenomena primarily because the method does not allow zero-background detection. Photoproduct concentrations produced by the low-energy pump pulses normally available from ultrafast laser oscillators and amplifiers are simply too small to show strong absorption signals against a large background. Nevertheless, the possibility that absorption techniques offer advantages complementary to those of other methods has motivated us to reexamine the absorption technique. These advantages include the ability to more easily determine absolute photoproduct yields, the ability to obtain broadband spectra using a probing supercontinuum, and the ability to detect intermediate species whose lifetimes are short compared with their fluorescence lifetimes. These advantages can be purchased at the cost of a laser system able to deliver high-energy ultrashort pulses (> 1 mJ), and by requiring the photochemical reaction under investigation to produce photoproducts with large absorption cross sections (> 10^-18 cm²) at high quantum yield (~1). Misewich et al.,¹ exactly so, have satisfied these demanding requirements and obtained femtosecond near-uv spectra of Tl atoms produced by photolysis of thallium halides at 308 nm.

© 1990 Optical Society of America